There are three major classes of digital modulation techniques used for transmission of digitally represented data: amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK). All the above digital modulation techniques convey data by changing some aspect of a base signal, the carrier signal, in response to a data signal. In the case of phase shift keying, the phase is changed to represent the content of the data signal. Therefore phase shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal, which is sometimes referred to as a carrier signal.
Any digital modulation scheme uses a finite number of distinct signals to represent digital data. In the case of phase shift keying, a finite number of phases are used. Each of these phases is assigned a unique pattern of binary bits. Usually each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. A demodulator, which is designed specifically for the symbol set used by the modulator, determines the phase of the received signal, and maps the phase back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal.
One convenient way to represent PSK schemes is on a constellation diagram. A constellation diagram shows the points in the Argand plane wherein the real and imaginary axes are termed the in-phase and quadrature axes respectively, due to their ninety degrees phase separation. Such a representation on perpendicular axes lends itself to straightforward implementation. The amplitude of each point along the in-phase axis is used to modulate a cosine (or sine) wave and the amplitude along the quadrature axis to modulate a sine (or cosine) wave.
In PSK, the constellation points chosen are usually positioned with uniform angular spacing around a circle. This gives maximum phase separation between adjacent points and thus the best immunity to corruption. They are positioned on a circle so that they can all be transmitted with the same energy. In this way, the moduli of the complex numbers they represent will be the same and thus so will the amplitudes needed for the cosine and sine waves. Two common examples are binary phase shift keying (BPSK), which uses two phases, and quadrature phase shift keying (QPSK), which uses four phases. Since the data to be conveyed are usually binary, the PSK scheme is usually designed with the number of constellation points being a power of two (2).
Owing to the simplicity of PSK, it is widely used in many existing technologies. For example, one popular wireless LAN standard, IEEE 802.11b uses a variety of different PSKs depending on the data rate that is required. BPSK is often employed for low cost transmitters, and is used in RFID standards such as ISO 14443, which has been adopted for biometric passports, credit cards, and other applications.
BPSK is the simplest form of PSK. It uses two phases which are separated by 180 degrees, and so is sometimes also referred to as 2-PSK. It does not particularly matter exactly where the constellation points are positioned, and in FIG. 1 they are shown on the real axis, at 0 degrees and 180 degrees, respectively. This modulation is the most robust of all PSKs, since it takes serious distortion to make the demodulator reach an incorrect decision.
One conventional circuit solution for modulating signal data onto a carrier signal is illustrated in FIG. 2. FIG. 2 illustrates a phase locked loop circuit 20, having a phase detector 22, a loop filter 24, a voltage controlled oscillator 26, and a divide by “n” circuit 28. The phase detector 22 compares a reference frequency signal (fref), for example, from a quartz oscillator with a feedback signal. Based on the phase comparison, the phase detector 22 generates an output signal to the loop filter 24, which generates a voltage control signal in response thereto. The voltage controlled oscillator receives the voltage control signal and generates an output frequency (fout) in response to the control signal, and provides such output frequency signal to the divider circuit 28 in a feedback loop of the circuit 20. The divider circuit divides down the output frequency signal and provides such divided-down signal back to the phase detector 22. Data is modulated onto the output frequency signal, in one example, by altering the divider value in the divider circuit 28 based on the desired data. This causes the feedback signal back to the phase detector to contain signal information which causes the output of the phase detector 22 to have differing comparison results based on the signal data. In the above manner, the signal data can be modulated onto the carrier signal, wherein the resultant modulated carrier signal is at the output of the VCO.